Internal combustion engines, and more specifically, reciprocating internal combustion engines are well known in the art. A conventional internal combustion engine typically includes a crankshaft, a crankcase disposed about the crankshaft, one or more cylinders exposed to the crankcase, a piston adapted to reciprocate within each cylinder, and a connecting rod drivingly coupling each piston to the crankshaft. The crankcase may be fixed to the frame of a vehicle such that the reciprocation of the pistons causes the crankshaft to rotate about an axis. Alternatively, the crankshaft may be fixed to the frame of a vehicle such that the reciprocation of the pistons causes the crankcase and cylinders to rotate about the crankshaft. Both of these configurations were commonly used to power aircraft in the early days of aviation. In particular, engines having the latter configuration with several cylinders radially disposed about the crankshaft were often referred to as “Gnome”-type engines.
Reciprocating internal combustion engines may be further classified as being spark-ignited (SI) or compression-ignited (CI). SI engines control the start of combustion by appropriately timing a spark plug that ignites an air-fuel mixture in the cylinder. The spark plug is often timed such that the start of combustion occurs when the piston reaches the top of the cylinder. To this end, the compression ratio of the engine must be kept relatively low in order to avoid engine “knock,” or the premature ignition of the air-fuel mixture. Traditional gasoline engines are typically of the SI type.
CI engines, on the other hand, control the start of combustion by compressing air within the cylinder and directly injecting fuel into the compressed air. Typically diesel fuel is injected into the compressed air, which is why traditional diesel engines are of the CI type. The increased pressure raises the temperature in the cylinder and eventually causes the air-fuel mixture to self-ignite. Such an arrangement requires CI engines to achieve higher compression ratios, and therefore, higher thermal efficiencies than comparable SI engines. In other words, traditional diesel engines are capable of more horsepower (BTUs) per volume of fuel when compared to their traditional gasoline counterparts. With the ever-increasing costs of gasoline, this aspect of a traditional diesel engine is particularly appealing to manufacturers and consumers of airplanes and other vehicles that consume large quantities of fuel.
Although several early attempts were made to develop a suitable CI or diesel engine for propeller-driven aircraft, there are many challenges associated with using these engines to power such aircraft. For example, combustion of the highly compressed air-fuel mixture in the cylinders can cause the pistons to generate significant shock “pulses” throughout the engine. These pulses can cause the engine to vibrate and thus lead to unsafe operating conditions. To reduce vibrations, CI engines are typically designed with heavier cylinders and crankcases to dampen the effect of the pulses. The additional weight, however, limits the aircraft's speed and altitude ability and thus has hereto before discouraged the use of CI engines in airplanes and other aircraft.
Maintenance difficulties are another challenge often associated with CI engines. For example, CI engines typically have a two-piece construction including a heavy cylinder head, a head gasket, and heat bolts coupling the cylinder head with the cylinder body. The cylinder head, head gasket, and head bolts are known to be common sources of failure because they are continuously exposed to the tremendous pressures associated with the cylinders.
In summary, the increased weight and maintenance challenges associated with CI engines has discouraged their use in the aircraft industry. Those in the industry abandoned attempts to capitalize on the advantages of CI engines and have instead relied upon SI engines due of their lighter weight. This is particularly true in the light to medium aircraft market. Moreover, over the past several decades there has been a significant trend towards using “jet-propelled” aircraft. Jet-propelled aircraft are typically powered by a gas turbine instead of the Si and CI reciprocating engines discussed above. Gas turbine engines generally experience much higher combustion temperatures than reciprocating engines and are adapted to deliver more power when compared to a reciprocating engine of the same weight.
Although jet engines have helped address some of the drawbacks associated with reciprocating engines, the solutions have come at an enormous cost to aircraft owners. For example, gas turbines often require complex designs and expensive materials because of the high combustion temperatures. Gas turbines can also be more costly to fuel than comparable reciprocating engines.
Therefore, there is a need for an improved compression-ignition engine that addresses the design challenges discussed above in order to provide an effective alternative to Si engines and an inexpensive alternative to gas turbine engines.